We study the organization and changes in organization of membrane components (lipids and proteins), both in the lateral and in the perpendicular direction. (1) We follow the insertion of proteins into a preformed lipid bilayer (either in the form of a planar bilayer or of a lipid vesicle), and study the actors which determine the protein's orientation. We measure permeability properties of lipid membranes to study: (a) mechanisms of ion transport; (b) properties of transport systems isolated from natural cell membranes; (c) mechanisms of cytotoxicity; (d) the effect of the membrane potential on the disposition of membrane proteins. (2) We have developed model systems in which fusion of phospholipid vesicles is induced CA2+, pH, and/or by such proteins as tubulin, clathrin, apocytochrome c and polylysine. We reconstitute viral spike glycoproteins into lipid bilayers and study the mechanism of pH-dependent membrane fusion mediated by those proteins. We study this fusion process using an assay involving resonance energy transfer between two fluorophores incorporated into the vesicle bilayer. (3) We observe lateral organization and movement of fluorescently - labelled molecules on cell surfaces by fluorescence microscopy. We study the mechanism by which asymmetry is maintained between apical and basolateral surfaces in epithelial cells.